Canopy temperature as an indicator of differential water use and yield performance among wheat cultivars

Agricultural Water Management, 18 (1990) 35-48

35

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

C a n o p y T e m p e r a t u r e as an I n d i c a t o r of D i f f e r e n t i a l W a t e r U s e and Y i e l d P e r f o r m a n c e among Wheat Cultivars

P.J. P I N T E R JR. 1, G. ZIPOLI 2, R.J. REGINATO'*, R.D. JACKSON', S.B. IDSO' and J.P. HOHMAN 1.*

USDA-ARS, U.S. Water Conservation Laboratory, Phoenix, AZ 85040 (U.S.A.) 2CNR, Institute of Environmental Analysis and Remote Sensing for Agriculture, Florence (Italy) (Accepted 14 July 1989)

ABSTRACT Pinter Jr., P.J., Zipoli, G., Reginato, R.J., Jackson, R.D., Idso, S.B. and Hohman, J.P., 1990. Canopy temperature as an indicator of differential water use and yield performance among wheat cultivars. Agric. Water Manage., 18: 35-48. Field experiments were conducted at Phoenix, Arizona to investigate the usefulness of canopy temperatures in screening wheat genotypes for water use and yield characteristics. Six spring wheat (Triticum aestivum L.) cultivars were grown under well-watered conditions and two deficit irrigation regimes. Canopy temperatures (To) were measured daily at 1045 h, 1345 h and 1545 h (MST) using hand-held infrared thermometers. Leaf stomatal conductances were obtained between 1245 h and 1400 h. Seasonal water use was estimated via a soil water budget approach. There were small but consistent differences in Tc among cultivars in the well-watered treatments. "Yecora", the warmest cultivar, had a mean midday Tc of 25.1 °C over the growth period from stem elongation until the hard dough stage of growth; "Siete Cerros" the coolest, was 23.3 ° C. Tc values were lower earlier and later in the day but all cultivars maintained the same ranking relative to each other. Cultivars with higher midday canopy temperatures under well-watered conditions used less water (r 2= 0.90; p < 0.01 ) and had lower stomatal conductances (r 2_- 0.86; p <0.01) than those with cooler temperatures. Grain yields for all cultivars were similar under well-watered conditions but varied considerably under water stress. Cultivars that were warmest when well-watered maintained the highest relative yields when exposed to deficit irrigation regimes (r2=O.78; p
36

P.J. PINTER JR. ET AL.

INTRODUCTION Soil moisture availability is the single most production-limiting factor in dryland agriculture. Consequently, much research has focused on increasing yield performance under sub-optimum water conditions via selection of drought-resistant germplasm from the gene pool. Hanson and Nelson (1980) examined agronomic, physiological and biochemical approaches which might be used in a breeding program for assessing genotypic sensitivity to drought. They suggested that an ideal screening test should be ( 1 ) rapid, accurate, and able to handle large numbers of samples during the season; (2) applicable early in the development of a plant; and (3) nondestructive. Few, if any, physiological screening tests meet these criteria. Remotely sensed infrared canopy temperatures, however, provide an efficient method for rapid, nondestructive monitoring of whole plant response to water stress (Idso et al., 1981; Jackson et al., 1981 ). A logical extension of this technology is the use of canopy temperatures to complement other methods currently used in screening genotypes for their resistance to drought (Kirkham, 1983; Sojka, 1985). The concept of using canopy temperatures measured with an infrared thermometer (IRT) to infer transpiration rates and moisture stress in plants was advanced more than a quarter of a century ago by Tanner (1963). Since then, IRTs have evolved from cumbersome, expensive laboratory instruments into very portable and affordable devices used routinely in agricultural field research. Potential applications for canopy temperature information have kept pace with the technological advances as well, ranging from yield prediction (Idso et al., 1977) and detection of plant disease (Pinter et al., 1979) to assessment of water status (Berliner et al., 1984), salinity stress (Howell et al., 1984 ), and evapotranspiration (Reginato et al., 1985 ). Commercially available IRTs are now available with dedicated microprocessors that translate canopy temperatures and ambient weather conditions directly into relative indices of plant stress that can be used, among other things, to optimize irrigations and applications of agricultural chemicals. For thermal infrared screening to have maximum value in plant breeding programs, sufficient variation in canopy temperature must exist between genotypes. The degree to which this occurs depends on the plant water status, growth stage, experimental techniques used and the evaporative demand of the environment. A study by Blum et al. (1982) found canopy temperature differences of various wheat and triticale strains were minimal when plants had a favorable moisture status but showed significant differences as water stress increased. Idso et al. (1984) examined the diurnal trend in canopy temperatures of the same six cultivars of spring wheat (Triticum aestivum L. ) that we report on here but their experiment was conducted just after the canopies had reached 100% soil cover and prior to head emergence. When well-watered, all

CANOPY TEMPERATURE AND WATER USE CHARACTERISTICS

37

cultivars displayed similar baseline behavior as vapor pressure deficit changed during the day. There are several schools of thought regarding how infrared thermometry might best be used in the selection process. The first suggests that temperatures should be measured under low soil moisture conditions, reasoning that plants with the cooler canopy temperatures are transpiring at higher rates and likewise are capable of relatively high photosynthetic rates, growth and yields. These plants would be following the "prodigal" strategy discussed by Passioura (1982). The investigations of Blum et al. (1982) mentioned above, seem to follow this tactic in their selection programs. Mtui et al. (1981) found certain maize (Zea mays L. ) hybrids were cooler and used more water, but had higher yields and water use efficiencies than their inbred parental lines. Gardner et al. (1986) studied five maize hybrids subjected to a gradient irrigation system. They reported that the yield reduction under water stress conditions was less for hybrids which maintained slightly cooler temperatures under stress. Plant water potential data published by Sojka et al. (1981) also imply a prodigal response for several of the wheat cultivars discussed in this article. The second strategy for using IRTs in genetic screening, reasons that plants should be measured under well-watered conditions. Those with higher temperatures transpire less, saving soil water for growth and reproductive efforts later in the season. This is Passioura's (1982) "conservative" response. Indeed, several recent studies show that canopy temperatures under well-watered conditions do provide clues to yield performance during drought. Singh and Kanemasu (1983) found as much as a 5 °C difference between afternoon canopy temperatures of ten pearl millet (Pennisetum americanum L.) genotypes growing in a non-stressed environment. The warmer genotypes had higher relative yields when grown in non-irrigated treatments. Kirkham et al. (1984) reported that inbred lines of drought-resistant maize growing under wellwatered conditions had about a 0.5 °C higher mean canopy temperature during the season than drought-sensitive lines. In another study the canopy temperatures of a large number of sorghum [Sorghum bicolor (L.) Moench ] and pearl millet genotypes were examined by Chaudhuri et al. (1986). The range in temperatures under well-watered conditions was very large; 14 °C for sorghum and 4°C for pearl millet. They observed that the warmer genotypes were able to produce viable seed at greater distances from a line source irrigation. Most recently, Hatfield et al. (1987) demonstrated that canopy temperatures could be used to identify water-conserving traits in commercial and exotic strains of cotton (Gossypium hirsutum L. ). Strains which were warmer earlier in the season had a reduced rate of soil water use and remained cooler in the latter part of the season. The strains which were warmer under irrigated conditions also produced more biomass under dryland field tests. Either the prodigal or conservative Tc response could prove useful in age-

38

P.J. PINTER JR. ET AL.

netic screening program depending upon the predominant patterns of precipitation in an area, the crop species involved, and its sensitivity to moisture stress at different stages of growth. The present study was designed to determine whether radiant canopy temperatures of well-watered cultivars of spring wheat were associated with: (1) water use and leaf conductance during nonwater stressed growing conditions; and (2) relative yield performance when deficit irrigation practices caused water stress at different growth stages. MATERIALS AND METHODS

Spring wheat (Triticum aestivum L.) was planted during mid-December, 1982 at Phoenix, Arizona. Six cultivars representing lines which had been selected for disease resistance and relatively high yield potentials under limited water conditions were obtained from CIMMYT at Ciudad Obregon, Sonora, Mexico. The cultivars were Ciano 79, Genaro 81, Pavon 76, Seri 82 (previously known as PMI HARI), Siete Cerros 66 (also known as 7 Cerros), and Yecora 70. Seed from each cultivar was sown at a rate of 300 seeds m -2 in separate flood irrigation basins averaging 12 by 19 m 2 in size. Plant rows were oriented in a N-S direction and were spaced 0.18 m apart. Convenient access to fields was facilitated by E - W boardwalks extending directly through the center of each plot. The soil was an Avondale loam [fine-loamy, mixed (calcareous), hyperthermic Antropic Torrifluvent].

Rainfall and irrigations Rainfall during the growing season totalled 143 mm; most fell during late winter before emergence of heads. Cultivars were subjected to three different irrigation treatments. Well-watered controls were grown under non-limiting water conditions throughout all growth stages. This required eight irrigations during the season with plants receiving an average 904 mm water (including rainfall; see Table 1). Plants in a second treatment (early stress) were irrigated three times, receiving an average 381 mm water. This treatment resulted in moderate to severe drought stress prior to and during heading that was temporarily relieved by a single 80 mm irrigation just after anthesis had occurred. Plants in a third treatment (late stress) were irrigated 4 times, receiving 583 mm water. They experienced light to moderate water stress prior to heading. A final 80 mm irrigation was given when 50% of the plants reached heading but severe stress later developed during the grain filling period. Volumetric soil water contents were measured using a neutron scattering technique. Observations were made three times each week in 0.2 m increments to a depth of 1.7 m. Seasonal water use by the plants was computed using a water balance approach over an assumed rooting depth of 1.1 m.

CANOPY TEMPERATURE AND WATERUSE CHARACTERISTICS

39

TABLE 1 C o m p o n e n t s of yield, water applied (includes a preplant irrigation and 143 m m total rainfall) a n d water used to a d e p t h of 1.1 m Cultivar

Stress treatment

Final grain yield (g m -2 )

1000kernel weight (g)

Heads m -2

Water applied (mm)

Water used to 1.1 m (mm)

Ciano

None Early Late

697 302 334

32.3 28.7 19.9

477 342 457

903 375 557

860 453 536

Genaro

None Early Late

749 427 444

34.4 27.7 25.7

547 429 490

901 383 543

812 479 545

Pavon

None Early Late

689 404 414

35.9 27.3 26.3

518 418 453

892 385 564

838 473 530

Seri

None Early Late a

710 371 461

41.0 30.4 30.3

393 351 400

884 379 628

799 457 623

Siete Cerros

None Early Late a

712 374 344

32.2 31.9 23.0

454 308 359

950 384 605

865 457 626

Yecora

None Early Late

712 478 579

43.7 37.0 36.4

450 437 448

897 382 602

769 468 541

~The Late stress t r e a t m e n t for Seri a n d Siete Cerros received one additional irrigation to enhance emergence after planting.

Plant measurements

Growth stage, biomass, leaf area and other pertinent agronomic parameters were determined from twice-weekly destructive plant samples taken in each field plot. Components of yield (final grain yield, 1000-kernel weight and number of productive tillers) were determined at maturity by hand-harvesting four 1 by 3 m areas near the center of each plot. Relative yields were computed for each cultivar by dividing the yields from the early and late stress treatments by the yield of the same cultivar in the well-watered control. Leaf conductances Estimates of stomatal conductance for abaxial and adaxial leaf surfaces were obtained with a Licor 1600 steady-state porometer*. Data were collected from *Trade names a n d company names are included for the benefit of the reader and do not imply any e n d o r s e m e n t or preferential t r e a t m e n t of the product listed by U.S. D e p a r t m e n t of Agriculture or the C N R of Italy.

40

P.J. PINTER JR. ET AL.

approximately 1245 h until 1400 h (MST) on most weekdays from 10 March 1983 until canopy senescence in mid- to late-May. Three uppermost sunlit leaves on separate plants of each treatment and cultivar were usually measured. In this article, we only include observations for plants in the well-watered treatment on days when skies were mostly clear and clouds did not interfere with the direct beam solar radiation.

Canopy temperatures On most non-raining days during the season, radiant canopy temperatures were measured three times using a calibrated, hand-held IRT with a nominal 4 ° field-of-view and 8-14 ~m bandpass filter: morning (1030-1100 h), midday (1330-1400 h) and afternoon (1530-1600 h, MST). A set of data was obtained with the instrument pointed straight down in the center of each plot from a height of about 1.5 m above the soil surface. Twelve measurements along an 8 m transect were used to calculate this nadir average temperature. A second data set was obtained by pointing the IRT obliquely at the wheat canopies at an angle of about 30 ° from the horizontal to minimize the amount of soil viewed by the sensor before 100% canopy cover was achieved. Six oblique measurements viewing towards the east and six towards the west were averaged for each cultivar. Air temperatures (1.5 m above the soil surface) were the average of a hand-held aspirated psychrometer and a ceramic wick thermocouple psychrometer located in the center of the experimental field plots. RESULTS AND DISCUSSION

Effects of irrigation treatments on yield The 700 to 750 g m - 2 grain yields obtained in the well-watered control treatments (Table 1 ) approached the potential yields expected for these lines when grown under conditions of minimal stress (CIMMYT, 1982, p. 65). Research by Sojka et al. (1981) included three cultivars in common with our study. They found similar high yields for control plots of Yecora, but lodging and leaf diseases may have contributed to the decline in yields they observed for Siete Cerros and Ciano. Components contributing to the final yield in our study varied with cultivar and treatment. The highest yielding cultivar in the wellwatered treatment, Genaro, also achieved the highest density of productive tillers but ranked intermediate in kernel weight. Yecora and Seri, on the other hand, had fewer tillers but produced much larger individual kernels, compensatory effects which resulted in yields only 5% lower than Genaro's. Yields were larger in the late stress than the early stress treatments for all except Siete Cerros (Table 1 ). This was not surprising since the plants in the late treatment received about 50% more water. When the yield was expressed

CANOPY TEMPERATURE AND WATER USE CHARACTERISTICS

41

relative to the control treatments, Yecora, the fastest maturing cultivar, appeared least affected by water stress. Early stress reduced the yield of Yecora by one third, whereas late stress depressed the yield by only 19%. Seri sustained a 35% yield loss when stress occurred late in the season. Yield reductions for other cultivars ranged from 40 to 57%, with Ciano and Siete Cerros performing the poorest. Water use

Water use in the well-watered treatment ranged from a minimum of 769 m m for Yecora to a maximum of 860 and 865 mm for Ciano and Siete Cerros, respectively (Table 1 ). The water applied to each cultivar in this treatment exceeded the amount used (estimated by the soil water balance). This indicates that some deep percolation may have occurred which was not accounted for. Plants in the late stress treatments used about the same amount as that applied by irrigation and rainfall, whereas those in the early stress treatments used approximately 22% more. Lea[ conductances

In the well-watered treatment, stomatal conductances measured in Yecora remained consistently below those in Siete Cerros (Fig. 1). Conductances of the other cultivars were usually between these extremes. With the exception of an abrupt (and unexplained) increase on day 89, we noted gradually declining conductances for all cultivars until the onset of anthesis in mid-April (about day 105). Then, although an irrigation on day 108 increased conductances briefly, values for Yecora and Seri {not shown) steadily decreased during grain I m

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42

P.J. PINTER JR. ET AL.

filling, whereas conductances of the other four cultivars remained higher. Both Siete Cerros and Ciano (not shown) maintained the two highest conductances during this period with values near 1.0 cm s - 1. Maintenance of higher transpiration when water became available after anthesis may explain why the 1000 kernel weight of Siete Cerros and Ciano differed so markedly depending on whether stress was imposed early or late in the grain development. Average leaf conductances were calculated from data collected under clear midday skies during the second to seventh day following each irrigation in the well-watered treatment. This precaution ensured optimum plant water status for the measurements. Distinct, cultivar-related conductance differences were observed in the absence of stress (Fig. 2). Yecora was most conservative (on a unit leaf area basis) in its use of water following irrigations. Conductances of Siete Cerros and Ciano were more than twice as large as Yecora's. These porometry observations are also consistent with the season long soil water use data discussed earlier; and, as Fig. 3 reveals, a highly significant correlation (r 2= 0.96) exists between these two parameters. Other studies, notably those by Sojka et al. (1981), Clarke and McCraig (1982) and Winter et al. (1988) have concluded that leaf diffusive resistances had little or no value in separating water use characteristics of wheat genotypes. However, season-long averages of this type of data might have hidden real differences because the amount of water available to each cultivar may have varied with the length of time after irrigation. 900

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CANOPY TEMPERATURE AND WATER USE CHARACTERISTICS

43

Canopy temperatures Midday oblique canopy temperatures (To) followed patterns typical for small grains grown in our climate (Fig. 4). These temperatures were influenced by ambient microclimate, percentage canopy cover, IRT view angle, plant phenology and plant and soil water status. During the spring months, increasing net radiation, accompanied by warmer air temperatures, resulted in a gradual increase in Tc. Morning values of T¢ were usually 2-4 ° C lower than midday Tc values, whereas afternoon observations were generally 1 ° C lower than midday Tc. Nadir and oblique T~ were very similar for newly emerged plants and also for 100% canopy cover conditions prior to heading, but differed by several degrees or more at other times. Nadir T~ values were higher with a partial canopy because of the larger proportion of warm soil viewed by the downward pointed IRT. After heading, however, oblique T¢ values were usually higher due to the larger contribution of warmer panicle and awn components (Hatfield et al., 1984). The Tc differences between cultivars in the well-watered irrigation treatment were usually 1-2°C. Although these differences were small and only slightly greater than the 0.5 °C stated accuracy of the IRT, they persisted throughout each time period and were consistent from day to day. This is emphasized in Fig. 5, where midday T~ minus Ta (air temperature) is shown for Siete Cerros and Yecora from March until late May, when the canopy cover was 100% and the differences between the cultivars were most pronounced. Yecora was 1 to 3 ° C warmer than Siete Cerros over much of this time period. 6 --~_.

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Fig. 4. Midday oblique canopy temperatures of Yecora and Siete Cerros wheat cultivars in the well-watered irrigation treatment. Fig. 5. The difference between midday oblique canopy temperatures, T¢, and air temperatures, Ta, at 1.5 m for Yecora and Siete Cerros wheat cultivars in the well-watered irrigation treatment. The scale in this figure is expanded over that shown in Fig. 4.

44

P.J. PINTER JR. ET AL.

The other four cultivars displayed intermediate thermal behaviors. Tc differences between cultivars also tended to be larger immediately following an irrigation and then became smaller as the time elapsed after an irrigation. This is consistent with observations of Singh and Kanemasu (1983) for pearl millet. We believe it can be explained by cooler (prodigal) cultivars depleting available water more rapidly than the warmer (conservative) lines. If the interval between irrigations had been extended longer, we surmise that Yecora would eventually become cooler than Siete Cerros. Such phenomena may help explain why Clarke and McCraig (1982) and Winter et al. (1988) report that canopy temperatures are unsuitable screening techniques for drought resistance in wheat. Because there were cultivar differences in water use rates and the stressed treatments were often irrigated on different days, the only valid intercultivar Tc comparisons were for days when we knew the plants had adequate water in the root zone. Thus, Tc and T ~ - Ta for the well-watered treatments were averaged over the same 2 to 7 day window following each irrigation that was used to compute average leaf conductances. Only data for clear days after 100% ground cover were used in the analysis. These selection criteria reduced possible comparison days to 13, 17 and 11 for the morning, midday and afternoon time periods, respectively. The preponderance of these data fell during the growth stages after heading. The results are summarized in Fig. 6 for all cultivars and time periods. The same ranking of cultivars occurred within each

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Fig. 6. Average oblique canopy temperatures measured at different times of the day for each wheat cultivar. Data were collected under clear skies and well-watered conditions existing from 2 to 7 days following irrigations. Vertical lines on bars indicate + 1 standard error. Fig. 7. Averages of each wheat cultivar's oblique canopy temperature minus the temperature ob-

served for the coolest cultivar, Siete Cerros, at different times of the day. Data were collected under clear skies and well-watered conditions existing from 2 to 7 days following irrigations. Vertical lines on bars indicate 1 standard error.

CANOPYTEMPERATUREAND WATERUSE CHARACTERISTICS

45

observation period: Yecora was always the warmest, while Siete Cerros was the coolest. The practical utility of the data shown in Fig. 6 is obviously limited by large morning to afternoon increases in temperature and the relatively large standard errors that were caused by the normal seasonal increase of Tc (Fig. 4). Subtracting T, from each To, had the effect of normalizing for day-to-day variation in ambient microclimate and reduced the standard error to a level where cultivar discrimination was possible (not shown). However, the absolute values of Tc minus T, were still strongly influenced by the time of day when the observations were made. To solve this problem, each cultivar's Tc was referenced to one another by subtracting the T~ of Siete Cerros, the coolest cultivar observed in our study. The results are shown in Fig. 7. With this approach, the time of day became less critical. Yecora remained warmer than the other cultivars and a full 1.4 to 1.8°C warmer than Siete Cerros. The greatest range in T~ difference occurred during the midday time period; but similar rankings and magnitudes were maintained among cultivars regardless of the time when measurements were taken. Nadir T~ observations showed similar differences between Yecora and Siete Cerros; however, the data were more variable and intermediate cultivars were not always ranked in the same order. The hypothesis that warmer canopy temperatures were associated with lower rates of evapotranspiration was strongly supported by significant relationships between midday oblique T~ differences and both the average leaf conductances of non-stressed plants {Fig. 8; r 2-- 0.86) and season-long water use in the wellwatered treatment (Fig. 9; r2= 0.90). 1.4

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P.J. PINTER JR. ET AL.

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Fig. 10. the average relative yields of each wheat cultivar in the early a n d late stress t r e a t m e n t s versus the average difference in midday oblique Tc between each cultivar a n d Siete Cerros under well-watered conditions. Vertical bars indicate range of relative yields for the two deficit irrigation treatments, r 2 = 0.78**.

The relationship between yield performance during water stress and radiant canopy temperatures of well-watered plants was determined by plotting the average relative grain yields of the water stressed treatments versus mean Tc differences between each cultivar and Siete Cerros. The resulting relationship (Fig. 10; r 2-- 0.78) was statistically significant at P < 0.05. The variation in these data was relatively high because the levels and timing of water stress were quite different for the two deficit irrigation treatments. The implication however, is clear. The cultivars with highest canopy temperatures under wellwatered conditions used the least amount of water for growth. This permitted larger relative yields when plants encountered drought stress later in the season. S U M M A R Y AND C O N C L U S I O N S

This study demonstrates the potential usefulness of radiant canopy temperature data for identifying water use characteristics of spring wheat cultivars. Consistent, cultivar-related differences in canopy temperatures were observed following irrigations when water availability was not limiting. Cultivars with the warmest canopy temperatures under well-watered conditions not only had the lowest leaf conductances and the lowest seasonal water use under normal irrigation practices but they also had the most favorable yield response when subjected to deficit irrigation conditions. Non-invasive approaches using infrared thermometry to rank genotypes according to canopy temperatures appear feasible for the efficient, nondestructive screening of large numbers of plants for water use characteristics and resistance to drought. We posit this technique will have the greatest potential utility in dry or semi-arid climates where relatively small differences in plant

CANOPYTEMPERATUREANDWATERUSE CHARACTERISTICS

47

transpiration are translated into canopy temperature differences of several degrees or more. To safeguard against differential rates of water use confounding the results, measurements should be made when all cultivars have equal access to ample soil water. An important precaution which must be observed, especially in small plots commonly used in breeding programs, is to minimize the influence of background soil temperatures on the thermal energy detected by the radiometer. However, the ability to use crops grown under well-watered conditions reduces the possibility that a poor stand or sparse canopy would complicate analysis. A thermal scanning device with its ability to distinguish temperatures of separate elements in the canopy would provide a sophisticated solution to this problem and should be explored in the future. This technique shows promise for identifying desirable traits among plants grown in breeders' nurseries. In practice, the temperature of an indicator genotype which possesses well-known water use, transpiration or yield response characteristics could be used for comparison. This would minimize the need for intensive monitoring of micrometeorological parameters, relax requirements for collecting data within a short midday time period, and reduce the necessity for extensive, time consuming measurements of physiological parameters. ACKNOWLEDGEMENTS

The authors wish to thank Drs. Joel Ranson and David Saunders, CIMMYT, Ciudad Obregon, Sonora, Mexico for supplying the seed for cultivars tested in this experiment; and wish to extend their appreciation to Dr. Jeff Baker, Ms. Elaine Ezra, Mr. Ron Seay, and Ms. Stephanie Johnson for field assistance during various phases of the experiment. Analysis of these data was partially supported by funding from IPRA-CNR, Italy.

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